1. Field of the Invention
The present invention relates to a circuit board for mounting a semiconductor chip (hereinafter referred to as a circuit board), wherein conductor bumps to be electrically connected to electrodes of the semiconductor carried on a substrate of the circuit board are formed on conductor pads formed on a surface of the substrate of the circuit board, and a method for inspecting the same.
2. Description of the Related Art
As examples of such a circuit board as described above, there are a first circuit board shown in FIGS. 14 to 16 and 17 to 19 and a second circuit board shown in FIGS. 20 to 22.
In the first circuit board, a circuit pattern 20 is formed on a surface of a substrate 10 thereof. A conductor pad 30 of a disc shape or other is formed on the circuit pattern 20. The substrate 10 is made of insulating material such as resin or ceramic. The circuit pattern 20 and the conductor pad 30 are formed by etching a copper foil adhered to the surface of the substrate 10 or by etching a metal skin applied to the surface of an insulating resin layer by a build-up system on the substrate 10.
A resist layer 40 comprised of insulating resin or the like is broadly formed on the surface of the substrate 10 for the purpose of preventing solder from sticking to the surface of the substrate 10.
The first circuit board shown in FIGS. 14 to 16 has an opening 50 in the resist layer 40, in which are consecutively exposed the conductor pad 30, the circuit pattern 20 adjacent thereto and a surface of a portion of the substrate 10 therearound.
The first circuit board shown in FIGS. 17 to 19 has an opening 50 in the resist layer 40, in which is exposed a central portion of the conductor pad 30.
A conductor bump 60 is formed on the conductor pad 30 exposed in the opening 50, which is of a sermispherical shape and constituted by conductive resin plated with solder or Au or mixed with conductive filler.
On the other hand, a metallized conductor pad 30 of a disc shape is provided on the surface of the substrate 10 made of ceramic or the like in the second circuit board. The conductor pad 30 is connected to the upper end of a column-like metallized conductor via 22 pierced through the substrate 10 in the thickness direction. The conductor via 22 constitutes a circuit pattern.
A resist layer 40 formed of insulating resin or the like is broadly formed on the surface of the substrate 10 for the purpose of preventing solder from sticking thereto. A circular opening 50 is formed in the resist layer 40 so that a central portion of the conductor pad 30 is exposed therein.
A conductor bump 60 is formed on the conductor pad 30 exposed in the opening 50 and is of a semispherical shape and is constituted by conductive resin plated with solder or Au or mixed with conductive filler.
In the prior art, when the first circuit board and the second circuit board are inspected, as shown in FIG. 16, 19 or 22, a height hl from the surface of the resist layer 40 to a top of the conductor bump 60 is measured by means of an optical detector using a laser beam or a visible ray (hereinafter referred merely to a "photometer") or a CCD (which is an abridgement of Charge Coupled Device) camera. Then, based on this measured value, a height h2 from the surface of the substrate 10 to the top of the conductor bump 60 or a height h3 from the upper surface of the conductor pad 30 to the top of conductor bump 60 are estimated. Next, based on this estimated value, a variation of heights of the conductor bumps is determined. Finally, based on this variation, it is determined whether or not the above-mentioned first and second circuit boards are acceptable.
The determination of whether or not the first and second circuit boards are acceptable is important because, if a large variation exists in height of the respective conductor bump 60 formed on the conductor pad 30 exposed in each of a plurality of openings 50 formed in the resist layer 40 in a matrix manner as shown in FIGS. 23 and 24, there might be a risk in that when a plurality of conductor bumps 90 formed on electrodes arranged in correspondence to the conductor pads 30 on one side of a semiconductor chip 80 in a matrix manner are pressed onto the plurality of conductor bumps 60 at a predetermined pressure so that both the conductor bumps 60, 90 are at once welded or pressbonded together, all of the conductor bumps 60 and 90 may not be electrically connected to each other in a reliable manner.
According to the above-mentioned determination in the prior art wherein the height h1 is first measured, then the heights h2 and h3 are estimated thereby, and finally the variation of the height of the conductor bump 60 is calculated from the estimated values, it is impossible to correctly obtain the actual variation of the height of the conductor bump 60.
This reason therefor is as follows.
There are two methods for forming the resist layer 40; one method wherein a resist liquid is coated broadly on the surface of the substrate 10, which is then dried at a high temperature, and another method wherein a dry film of resist formed in a sheet form is adhered onto the surface of the substrate 10, which is then dried at a high temperature.
The surface of the resist layer 40 formed broadly on the surface of the substrate 10 by these methods is not evenly flat but tends to undulate up and down, causing a variation in height everywhere on the surface of the resist layer 40.
Therefore, it impossible to use the surface of the resist layer 40 formed on the surface of the substrate 10 in the above-mentioned manner as a reference surface for the correct estimation of the height h2 or h3.
To solve such a problem, two methods are proposed; a first one wherein the height h2 is directly measured by an optical detector or a CCD camera while using, as a reference surface, the surface of the substrate 10 in the vicinity of the conductor bump 50 exposed in the opening 50, and a second one wherein the height h3 is directly measured by an optical detector or a CCD camera.
According to the first or second method, it is believed that the variation f a height of the conductor bump 60 formed on the substrate 10 is correctly determined.
The first and second methods, however, could not be applied to the above-mentioned first or second circuit board of the prior art.
The reason why the height h2 from the surface of the substrate 10 to the top of the conductor bump 60 could not be measured by the first method is that a gap between the conductor bump 60 and the inner wall of the opening 50 is too narrow as shown in FIGS. 15 and 16, and therefore a width of the surface area of the substrate 10 exposed in the gap is also too small. When one wishes to directly measure the height h2 from the surface of the substrate 10 in the vicinity of the bump 60 exposed in the opening 50 to the top of the conductor bump 60 by the first method while using the optical detector, it is necessary to emit a laser beam or a visible ray from the optical detector disposed above the resist layer 40 onto the surface of the substrate 10 and reflect or diffuse the same to be directed to the optical detector. Such a reflected or diffused ray, however, is difficult to make incident upon the optical detector due to the interference with the conductor bump 60 or the resist layer 40. on the other hand, if a CCD camera is used to directly measure the height from the surface of the substrate 10 in the vicinity of the bump 60 exposed in the opening 50 to the top of the conductor bump 60 by the first method, it is also difficult to focus the CCD camera on the surface of the substrate 10 of a narrow width.
To enable the height h2 to be directly measured by the first method while using an optical detector, a CCD camera or the like, it is suggested to provide a large opening 50 in the resist layer 40 so that the surface of the substrate 10 is broadly exposed in the vicinity of the conductor bump 60 within the opening 50.
In such a case, however, there is a wide space between the conductor pad 30 exposed in the opening 50 and the inner side of the neighboring resist layer 40, whereby it is impossible to properly form a generally semispherical conductor bump 60 having a predetermined height on the conductor pad 30 in the opening 50 while effectively using the neighboring resist layer 40. Accordingly, the formation of large opening 50 in the resist layer 40 as described above could not be adopted.
The reason why the height h3 could not be measured by the second method is that the laser beam or a visible ray cannot pass through the conductor bump 60 to reach the upper surface of the conductor pad 30 beneath the former. Also, the laser beam or the visible ray reflected or diffused on the conductor pad 30 cannot pass through the conductor bump 60 to reach the optical detector or the CCD camera disposed above the resist layer 40.